Long story short, internal requests inside fly.io don’t play nice with multi-process apps.
Essentially the request is sent to one of the processes randomly and currently there is no way to control which process gets your internal request.
If you want this to work you’ll need to go through the proxy layer by using the public dns.
I would recommend splitting your multi-process app into multiple single-process apps as currently multi-process apps deploy separate VMs for each process anyway.